Actually, you can extend the idea and create a bonafide caste system: the top layer has nobles and a few "servants" (read: porters). Raw materials get dropped into various holes, and two levels down finished goods magically appear. The workers trapped in the middle layer have to fend for themselves for food.
I think it would be more Dorf-like to have the higher castes living deeper, with raw materials being dumped from above to be used below:
Surface Level: The Undesirables. Tree Cutters, Surface Farmers, Surface Fishers, Surface Hunters, Surface Herbalists. Live in small wooden huts, if they are lucky. PRODUCES: Wood, Food, Hides, Bones, Shells.
Depth 1: Trade Depot, Underground Farming and Carpentry. The Bookkeeper, sullied by trade with outsiders, lives here. CONSUMES: Wood. PRODUCES: Beds, More Food, Hides, Bones.
Depth 2: The Kitchens, Breweries, Leatherworks and Craftsdwarf's Workshops. CONSUMES: Food, Bones, Hides, Shells. PRODUCES: Crafts, Meals, Booze. NOTE: Alcohol is only permitted to Depth 2+ Dwarves. Pity the surfacers!
Depth 3: Masonry and Mechanisms. PRODUCES: Furniture, Stone Blocks, Mechanisms, etc. Minor nobility lives here.
Depth 4: Glass and Metal. PRODUCES: Glass, Weapons, Armor. The military elite resides here, along with higher nobility.
Depth 5: The Crown Jewelry and artifact treasure vault. There is never more than one Jeweler at a time; the rest are made into porters along with the soap smiths and cheese makers. The King and Queen live here, together with the royal guard.
Porters use special segregated tunnels (that can be dumped into from production levels, but can't be used to move goods the other way) to transport items to be traded to the outside.
There are special service tunnels that are usually closed that can allow military units to reach higher levels in order to put down uppity peasants. The levers that open these are securely located in the royal guards' headquarters, protected from any mad craftsdwarves or skulking filth.